Yoshimori Honkura

A numerical dynamo benchmark

We present the results of a benchmark study for a convection-driven magnetohydrodynamic dynamo problem in a rotating spherical shell. The solutions are stationary aside from azimuthal drift. One case of non-magnetic convection and two dynamos that differ in the assumptions concerning the inner core are studied. Six groups contributed numerical solutions which show good agreement. This provides an accurate reference standard with high confidence.

Magnetotelluric imaging of fluids in intraplate earthquake zones, NE Japan back arc

Intraplate earthquake zones in the back arc of NE Japan were imaged by wide-band magnetotelluric (MT) soundings. The 90km long MT profile of 34 stations extends over the two topographic features, the Dewa Hills and the Ou Backbone Range, which were uplifted by thrust faults. MT data show two-dimensionality and strong TE/TM anisotropic responses at the periods around 100 s. After tensor decompositions with regional strike of N12°E, two-dimensional inversion was carried out where static shift was also a model parameter. The final model is characterized by conductive blocks in the mid-crust to account for the anisotropic responses. Correlation of the conductors to the seismic scatterers and to the low velocity anomalies suggests that the conductors represent fluids. High seismicity clustering near the rims of conductors suggests that the intraplate seismicity results either from the migration of the fluids to less permeable crust or from local stress concentration near the structural boundary.

Preliminary results of multidisciplinary observations before, during and after the Kocaeli (Izmit) earthquake in the western part of the North Anatolian Fault Zone

On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

Mid-crustal electrical conductors and their correlations to seismicity and deformation at Itoigawa-Shizuoka Tectonic Line, Central Japan

An active fault segment at the northern Itoigawa-Shizuoka tectonic line (ISTL), Central Japan, which will potentially cause M8-class intraplate earthquake, was imaged by wide-band magnetotellurics. Three parallel profiles across ISTL revealed along-strike variation of the resistivity structure. Three resistivity models commonly showed the thickening conductors in the upper crust to east of ISTL which imply the heavily folded Miocene sediments with maximum thickness of several kilometers. Thus the upper crustal structure seems two-dimensional throughout the segment. We found mid-crustal conductors, top of which correlate well with the cutoff depth of seismicity. The seismicity clusters mainly in the resistive crust that is underlain by the mid-crustal conductors. This implies the local distribution of fluids below the brittle-ductile boundary and suggests that the fluid migration into resistive zone is triggering earthquakes. However, the distribution of these mid-crustal conductors is not consistent with the strike of ISTL, but rather it is better correlated with the negative dilatation anomaly inferred from GPS. This suggests the weakening of the crust by the existence of fluids.

Two electrical conductors beneath Kusatsu-Shirane volcano, Japan, imaged by audiomagnetotellurics, and their implications for the hydrothermal system

Kusatsu-Shirane volcano, Japan, is known for its active phreatic eruptions. We have investigated its hydrothermal system by conducting audio-magnetotelluric soundings at 22 stations along a profile that extends across the volcano. The final two-dimensional model is characterized by two conductors. One is a 300- to 1000-m-thick conductor of 1–10 Ωm, which is located on the eastern slope and covered with 200-m-thick resistive layers of Kusatsu-Shirane lava and pyroclastics. This conductor indicates the presence of a Montmorillonite-rich layer of Pliocene volcanic rocks that may function both as an impermeable floor for the shallow fluid path from the peak to the hot springs to the east and as an impermeable cap for the deeper fluid path from the summit region to the foot of the volcano. The second conductor is found at a depth of 1–2 km from the surface, at the peak of the volcano, and its resistivity is as low as 1 Ωm or less. This low resistivity can be explained by fluids containing high concentrations of chloride and sulfate which were supplied from the magmatic gases. Micro-earthquakes cluster above this conductor, and the cut-off of the earthquakes corresponds to the top of the conductor. This conductor infers the presence of the fluid reservoir, and the upward release of these fluids from the reservoir through the conduit presumably triggers the micro-earthquakes at the peak area of the volcano. Crustal deformation modeling using GPS and leveling data of the past 10 years revealed that the center of the deflation coincides with the top of the second conductor, indicating that the fluid reservoir itself can be hosting the deformation.

Geological evidence for the last two faulting events on the north Anatolian Fault zone in the Mudurnu Valley, western Turkey

Abstract Recurrence intervals of large earthquakes produced by slip on the North Anatolian fault zone have been inferred mainly from historical records, and included much ambiguity. It is therefore important to date individual earthquakes using geological and -geomorphological methods. We excavated an exploratory trench across the surface rupture zone associated with the Mudurnu Valley, western Turkey, earthquake of July 22, 1967, and revealed evidence for the last two faulting events including the 1967 earthquake. On the basis of the radiocarbon dates of sediments in the trench, we estimate most conservatively that the penultimate event occurred after 1480 A.D., but believe it likely that the event occurred shortly after 1650 (±20) A.D. The penultimate event is likely to be correlated with the Anatolian earthquake of August 17, 1668, which caused severe damage along the North Anatolian fault zone for about 600 km from Bolu to Erzincan.

Resistivity imaging across the source region of the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

Across the source region of the 2004 Mid-Niigata Prefecture earthquake, wideband magnetotelluric (MT) survey was performed just after the onset of the mainshock. Owing to the temporal stop of the DC powered railways around the area together with intense geomagnetic activity, we obtain MT records with excellent quality for both short and long period data, as long as 10,000 s. Two dimensional regional strike is evaluated with the aid of the Groom-Bailey tensor decomposition together with induction vector analysis. As a result, N15°W is determined for the strike. This strike is oblique to the local geological trend and also to the strike of the main shock source fault together with aftershock distribution of N35°E. Two dimensional resistivity structure is determined with the aid of an ABIC inversion code, where static shift is considered and estimated. Characteristics of the structure are as follows. (1) About 10 km thick sedimentary layer exists on the top. (2) A conductive body exists in the lower crust beneath the source region. The mainshock occurred at the boundary of the conductive sedimentary layer and a resistive basement beneath it and aftershocks occurred in the sedimentary layer. From geological studies, it is reported that the sedimentary layer was formed in the extensional rift-structure from Miocene to Pleistocene and has been thickened by compressional tectonic regime in the late Quaternary. Interstitial fluids or clay minerals, which reduce the sedimentary layer resistivity, control the reactivation of the normal fault as the mainshock thrust fault and aftershock activity. The second conductive body probably indicates existence of fluids in the depths as well. Such a conductive layer in the lower crust was also revealed by previous MT experiments along the Niigata-Kobe Tectonic Zone and probably plays a main role in concentration of strain rate along the zone.

Partial melting and electrical conductivity anomalies beneath the Japan and Philippine seas

Abstract Geomagnetic variation anomalies observed in the central part of Japan can be accounted for by highly conducting layers having a conductivity of 0.5 S/m or thereabouts beneath the Japan and Philippine seas. High temperature may be a possible cause of such layers if we take into account that temperature amounts to 1,000°C at a depth of 40 km beneath the areas of high heat flow in the Japan and Philippine seas. It seems difficult, however, to account for a conductivity of 0.5 S/m which is unusually high at a depth of 50 km or so by means of high temperature only. We can interpret such a high conductivity in terms of partial melting of peridotite in the upper mantle, provided highly conducting molten material is interconnected. This is in good agreement with the interpretation of low-velocity zones beneath the Japan and Philippine seas in terms of a partial melting of 4–6%.

Resistivity structure across Itoigawa-Shizuoka tectonic line and its implications for concentrated deformation

We investigated the deep crustal resistivity structure across Itoigawa-Shizuoka Tectonic Line (ISTL), one of the most dangerous active intraplate faults in Japan, by use of wide-band magnetotelluric (MT) method. The 28 MT stations were aligned perpendicular to the ISTL. A two-dimensional model was created in transverse magnetic (TM) mode where electric currents flow in N60°W-N120°E directions. The model showed good correlations with the surface geology. In particular, we found a thick (∼6 km) surface conductor to the east of ISTL which corresponds to the heavily folded sedimentary layer. The Japan Alps to the west of the ISTL is characterized by the resistive upper crust, where the pre-Tertiary rocks crop out. The Japan Alps is underlain by a conductor below 15–20 km depth, which is consistent with the low seismic velocity anomaly. We also found a localized shallow conductor corresponding to the Mt. Tateyama volcano. The most important feature is the conductor in the mid-crust directly under the area of active folding to the east of the ISTL. This may imply a localized zone of fluids because of the enhanced porosity in a shear zone. The recent seismicity clusters in the resistive crust underlain by the conductor, and this suggests the fluid involvement in earthquake generation processes.

SOME CHARACTERISTICS OF MAGNETIC FIELD BEHAVIOR IN A MODEL OF MHD DYNAMO THERMALLY DRIVEN IN A ROTATING SPHERICAL SHELL

Abstract A model of three-dimensional self-consistent MHD dynamo in a rotating spherical shell is investigated through numerical simulations, in which a fully spectral scheme is used to guarantee precise computation in space. Generation mechanisms of the magnetic field and the structures of the magnetic and the velocity fields are then examined with emphasis on a fundamental process in the Earths core. It turns out that the magnetic field is confined in forward convective columns with respect to the drift direction of convection pattern. Detailed examination of generation mechanism of the magnetic field indicates that a strong toroidal magnetic field is generated by shear motion near the equatorial region close to the outer surface, whereas the poloidal magnetic field is rather maintained by fluid motion related to convective columns. Even when the magnetic and the velocity fields vary considerably, this relationship holds with some disruption during disordered states, and time variations of the magnetic field always show a time lag behind those of the velocity field. Although the so-called weak-field regime is investigated, fundamental phenomena shown in this paper should be of importance for understanding of the Earth and planetary dynamo processes.